You might have something that runs on the battery that is on.
Most of our fridges have a switch that removes door moisture.
It is located around the door frame/door. Check that first. If you can not find it turn the fridge off and see if the monitor recovers. It will not do it right away.. 15 min or so it should. Give us the model number/make of your fridge and we will tell you where it is located.
Most of us never use that option so it remains off.
Is your fridge a 3 way... 120vac/12vdc/propane? Turn the fridge off and see if the voltage recovers after 15 min or so.
You do need a volt meter. Simple to use and you can play with it in DC voltage mode with a any small battery...
Jim
